Optical glass having anomalous dispersion properties

ABSTRACT

An optical glass having an anomalous dispersion property in the short wavelength region, a high Abbe number of greater than 52, a high stability of devitrification and an excellent acid resistance and consisting of. P2O5 44.0 to 68.0%, B2O3 5.0 to 21.0%, Al2O3 1.0 to 8.0%, BaO 4.0 to 33.0%, MgO 1.0 to 14.5%, ZnO 0 to 10.5%, SrO 0 to 12.0%, CaO 0 to 7.0%, La2O3 0 to 9.0%, ThO2 0 to 7.0%, TiO2 0.5 to 4.2% and As2O3 0 to 3.5%, by weight is disclosed.

llnited States Patent [191 Izumitani et a1.

[ Mar. 19, 1974 OPTICAL GLASS HAVING ANOMALOUS DISPERSION PROPERTIES [75] Inventors: Tetsuro Izumitani; Isao Masulda,

both of Tokyo, Japan [22] Filed: July 24, 1972 [21] Appl. No.: 274,384

[30] Foreign Application Priority Data July 23, 1971 Japan 46-55113 [52] US. Cl. 1106/47 Q [51] Int. Cl. C03c 3/16 [58] Field of Search 106/47 Q, 47 R [56] References Cited UNITED STATES PATENTS 3,490,928 1/1970 Brewster et al 106/47 Q FOREIGN PATENTS OR APPLICATIONS 1,339,339 8/1963 France 106/47 0 1,089,935 9/1960 Germany 106/47 Q Primary Examinerl lelen M. McCarthy Attorney, Agent, or Firm-Sughrue, Rothwell, Mion, Zinn and Macpeak [5 7] ABSTRACT An optical glass having an anomalous dispersion property in the short wavelength region, a high Abbe number of greater than 52, a high stability of devitrification and an excellent acid resistance and consisting of.

P 0 44.0 to 68.0%, B 0 5.0 to 21.0%, A1 0 1.0 to 8.0%, BaO 4.0 to 33.0%, MgO 1.0 to 14.5%, ZnO 0 to 10.5%, SrO 0 to 12.0%, G10 0 to 7.0%, La O 0 to 9.0%, Th0 0 to 7.0%, TiO 0.5 to 4.2% and A8203 0 to 3.5%, by weight is disclosed.

2 Claims, 1 Drawing Figure PATENTED MAR 1 9 I574 ogo OFIICAI. GLASS HAVING ANOMALOUS DISPERSION PROPERTIEfi:

BACKGROUND OF THE INVENTION:

1. Field of the Invention This invention relates to an optical glass having the abnormality of a dispersion property in the short wavelength region, a high Abbe number of greater than 52, a high stability to devitrification and a favorable acid resistance.

2. Description of the Prior Art In the production of a super-achromat, super achromatism cannot be obtained by assembling normal dispersion glasses wherein the relation between the Abbe number (11 and the partial dispersion ratio (P on the g-h line is linear but are obtained by assembling anomalous dispersion glasses having a relation between the Abbe number and the partial dispersion ratio which is different from that of the normal dispersion glass. Especially, an optical glass having an anomalous dispersion property in the range near the ultraviolet region is required to obtain super-achromatism, since the sensitivity spectrum of the photosensitive material and that of the photoelectric tube are high at the short wavelength side. The partial dispersion ratio is indicated by n,,n,/n -n The inventors have already applied for a patent on a TiO containing anomalous glass belonging to the above glass type, as a Japanese Patent Application No. 17757/ 70. This glass, however, has poor stability to de-.

vitrification where the T10 content is high, and accordingly, it is not particularly suitable for practical USE.

SUMMARY OF THE INVENTION Therefore, an object of this invention is to overcome the above defect and to provide an improved glass composition having a high stability to the devitrification. According to this invention, B is added to a TiO containing phosphate glass to improve the stability to devitrification, and further La O and Th0: are

added to a base glass composition to compensate for BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING The accompanying drawing shows the relationship between the 11,, and the P for a normal dispersion glass and the relationship for the glasses of this invention.

DETAILED DESCRIPTION OF THE INVENTION The P 0 which is the main component of a phosphate glass, exhibits a basic absorption in the short wavelength region apart from the visual light zone, so that the dispersion curve of refractive index plotted against wavelength for the phosphate glass is gentle and the Abbe number of the phosphate glass is high. Where the P 0 content exceeds 60.0 percent by weight, the stability to devitrification is steeply reduced, whereas where the P 0 content is lower than 414 percent the Abbe number is too low and the acid resistance is decreased.

TiO having a strong absorption property in the region near the ultraviolet region, imparts to the glass the anomaly of dispersion in the short wavelength region.

The greater the amount of P 0 is, the greater the effect of TiO; is increased, but an excess amount of TiO reduces the Abbe number of the resulting glass and lowers the stability to devitrification of the glass. The preferable range of the TiO content ranges between 0.5 to 4.2 percent.

A1 0 and E 0 especially the latter, are effective to compensate for the reduction of the stability to devitrification arising as a result of the addition of TiO The preferable range of B 0 and that of Al O are 5.0 to

21.0 percent and 1.0 to 8.0 percent, respectively.

are favorable glass modifiers for the phosphate base glass of this invention. The BaO improves remarkably the stability of the glass to devitrification, and the preferable range is 4.0 to 33.0 percent. 1.0 to 14.5 percent of MgO prevents the coloring of the glass blue due to Ti ion and also improves the stability to devitrification. 0 to 10.5 percent of ZnO brings out the same effect as that of MgO. SrO increases the stability to devitrification for the glass having a relatively low phosphate content, and the preferable range is 0 to 12.0 percent. 0 to 7.0 percent of CaO improves the chemical durability of the glass. La O and Th0;, a part of which can replace BaO, improve remarkably the acid resis tance without reducing the Abbe numer and the anomalous dispersion property. The effect of La O and that of Th0 are to restore the acid resistance, which is reduced by the addition of a large amount of BaO, to a level suitable for practical use. For example, if 6 percent of Ba() is substituted by La O the weight loss (wt.%) of the glass due to the acidic attack can be reduced to half. ThO exhibits a similar effect as that of La O but not as strongly. The preferable range of the La O content and that of the ThO content are 0 to 9.0 percent and 0 to 7.0 percent, respectively. If the La O content and ThO exceed the above restricted ranges, both the anomalous dispersion property and the stability to devitrification are reduced. As O also suppresses the detrimental blue coloring due to the Ti ion, which is caused by adding excess amounts of TiO, to the phosphate glass, and the preferable amount is 0 to 3.5 percent.

Preferred ranges of the amounts for the components of the glass of this invention are P 0 46.0 to 60.0%, B 0 5.0 to 15.0%, A1 0 2.0 to 5.0%, BaO 7.0 to 25.0%, MgO 5.0 to 12.0%, ZnO 0 to 10.0%, SrO 0 to 6.0%, CaO O to 6.0%, La O Th0, 2.0 to 7.0%, TiO

TABLE 1 Mt. percent) Specimen number P20 B A120; BaO MgO CaO L320: Th0 T102 A5203 Table 2 shows the various physical properties of the glass compositions listed in Table 1.

The liquidus temp. (Lt) was determined by measuring the highest temperature at which devitrification was caused in the glass i.e., 1.0 to 2.0 mm diameter granules of the glass to be tested were placed on a Pt plate, which was charged in a furnace having a temperature gradient therein from 700C to 1,200C after being in the furnace for about 40 min., the glass granules were inspected using a microscope to observe at which temperature devitrified granules occurred.

The acid resistance was determined using the method of measuring the chemical durability of the optical glass (powder method)" in the Japanese Optical Glass Industrial Standard. This acid resistance is classitied into five grades as follows;

Grade Weight Loss due to Acidic Attack 1st Class 0.21 0.35 2nd Class 0.36 0.65 3rd Class 0.66 1.20 4th Class 1.21 2.20 5th Class 2 2.21

In the FIGURE, the straight line (A-B) shows the relationship between the v and the P for the normal dispersion glass, whereas the dots show the relationship for the anomalous dispersion glass specimens according to this invention. The number on each dot corresponds to the specimen number in Table land of the dots l to 12 lie in the zone to the right of the straight line (A-B), and this means that the glass specimens of this invention have the anomalous dispersion properties.

The glass specimens listed in the Tables were produced by mixing orthophosphoric acid, metaphos O phates of alkaline earth metals, carbonates, and hydroxides together, fusing the resulting mixture at l,280 to 1,350C for 30 min to 2hr, agitating and refining the mixture, casting the clarified mixture into a mold and cooling the obtained glass slowly.

It is apparent from the above examples that an optical glass having an Abbe number of greater than 52 and a high anomalous dispersion property in the short wavelength region, a sufficient stability to devitrification and an excellent chemical durability can be obtained according to this invention.

While this invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the scope of this invention.

Therefore, the appended claims are intended to cover all such equivalent variations as comming within the true spirit and scope of this invention.

What is claimed is:

1. An optical glass having an Abbe number greater than 52 and a high anomalous dispersion property in the short wavelength region, stable to devitrification and having excellent acid resistance, consisting of by weight, P 0 44.0 to 68.0%, B 0 5.0 to 21.0%, A1 0 1.0 to 8.0%, BaO 4.0 to 33.0%, MgO 1.0 to 14.5%, ZnO 0 to 10.5%, SrO 0 to 12.0%, CaO O to 7.0%, La o 0 to 9.0%, Th0; 0 to 7.0%, T10 0.5 to 4.2%, and A5 0 0 to 3.5%.

2. The anomalous dispersion optical glass of claim 1, wherein said glass consists of, by weight, P 0 46.0 to 60.0%, B 0 5.0 to 15.0%, A1 0 2.0 to 5.0%, Ba 0 7.0 to 25.0%, MgO 5.0 to 12.0%, ZnO 0 to 10.0%, SrO O to 6.0%, CaO 0 to 6.0%, La O Th0;- 2.0 to 7.0%, TiO 1.0 to 3.0% and A5 0 0 to 2.0%. 

2. The anomalous dispersion optical glass of claim 1, wherein said glass consists of, by weight, P2O5 46.0 to 60.0%, B2O3 5.0 to 15.0%, Al2O3 2.0 to 5.0%, Ba O 7.0 to 25.0%, MgO 5.0 to 12.0%, ZnO 0 to 10.0%, SrO 0 to 6.0%, CaO 0 to 6.0%, La2O3 + ThO2 2.0 to 7.0%, TiO2 1.0 to 3.0% and As2O3 0 to 2.0%. 